Intelligent switch for connecting power to a load

ABSTRACT

A switching circuit includes a microchip which, in response to a signal from a signal switch, controls the operation of a power switch which, when closed, connects a load to a battery. The microchip can monitor the status of the battery and control the power switch to ensure optimum operation of the load and optimum usage of the energy in the battery. The microchip can control the connection of the load to the battery in different ways according to the manner of operation of the signal switch.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national phase filing under 35 U.S.C. 371 ofInternational Application No. PCT/ZA01/00081, file Jun. 13, 2001.

BACKGROUND OF THE INVENTION

The invention relates generally to an intelligent switch which issuitable for controlling the use of a battery driven device such as aflashlight, toy, motor or the like and, more particularly, is concernedwith an intelligent electrical device of the kind described in theapplicant's international application No. PCT/ZA99/00107.

In the PCT application mechanical switches function as aman-machine-interface (“MMI”) between the device and an operator. TheMMI functions are controlled by very low current signals using touchpads, carbon coated membrane type switches or similar mechanisms. Amicrochip is responsive to input signals from the MMI and can, accordingto application, be used in various ways. For example with a flashlightthe MMI may control the on/off operation of the flashlight, cause theflashlight to be turned off after a predetermined time, provide anindication of battery strength, or enable the flashlight to be operatedwith a desired flashing sequence. Various other features can also beachieved through the judicious use of the microchip and reference ismade to the specification of the PCT application for a furtherdescription of such features.

The applicant is aware of a number of systems which provide anindication of the charge left in a battery.

Osterhout et al (U.S. Pat. No. 4,876,632) describes a circuit whichdetects the closed circuit voltage of a battery pack used in aflashlight. Based on predetermined reference voltages the battery lifeis shown in one of three categories. The circuit is activated by anon/off switch of a flashlight and does not function when the flashlightis off. This is to ensure that there is no power consumption when theflashlight is off. If however the circuit should function when the lightis off the open circuit voltage of the battery will be measured and thiswill give a misleading indication of the available battery life.

Mallory (U.S. Pat. No. 4,499,525) and Weber (U.S. Pat. No. 5,821,697)relate to the provision of constant illumination by flashlights. Muchcan be gained in terms of quality of light and operating life of lightbulbs by maintaining constant power through a light bulb. It is to benoted that in most instances the voltage which is supplied by a batteryvaries quite extensively over its usable life. Mallory makes use of anRC filter to determine an effective current and time measurement. Basedon component selection the duty cycle of a current delivered to a lightbulb at a specific voltage can be determined.

Weber uses a different technique wherein power is dissipated in avariable resistor or transistor element. This approach is wasteful ofpower.

SUMMARY OF THE INVENTION

The present invention is concerned with various modifications andimprovements which can be made to the microchip and to the embodimentsof the invention described or claimed in the specification of the PCTapplication. To the extent which may be necessary for an understandingof the present invention the disclosures in the specification of the PCTapplication are to be read in conjunction with this specification and,where applicable, such disclosures are to be deemed to be incorporatedinto this specification.

According to a first aspect of the invention there is provided aswitching circuit for controlling the supply of power from anexhaustible power source to a load, which includes a control circuit, apower switch which is controlled by the control circuit and which, whenclosed, connects the power source to the load, and at least one signalswitch which is connected to the control circuit, and wherein thecontrol circuit provides at least one function selected from thefollowing:

-   (a) the control circuit causes the power switch to be operated in a    manner which is dependent on the operation of the signal switch,-   (b) the control circuit compares the voltage of the power source,    when the power source is connected to the load, to at least one    reference voltage thereby to provide an indication of the status of    the power source,-   (c) the control circuit monitors the voltage of the power source and    when the power source voltage drops below the operating voltage of    the control circuit, the power switch is controlled by the signal    switch to connect the power source to the load,-   (d) the control circuit monitors the voltage of the power source and    the power switch is latched on or off, according to requirement,    when the power source voltage drops below the operating voltage of    the control circuit, and-   (e) the control circuit controls the duty cycle of the power switch,    in a manner which is dependent on the voltage of the power source,    to provide a substantially constant supply of power by the power    source to the load.

A specific sequence of operations of the signal switch may beinterpreted to provide for the application of power to the load for anindefinite period, ie. effectively permanently, until terminatedaccording to a different criterion.

In another embodiment if the signal switch is held on for an extendedperiod the power switch may be operated in such a way that a dimming orreduced power operation results.

In a variation the power switch is closed for a predetermined periodwhich is a function of the duration of a time period for which thesignal switch is operated.

According to a second aspect of the invention the control circuitincludes the ability to monitor the voltage level of the exhaustiblepower source (ie. a battery) which is connected to the load and when thevoltage level drops below the operating voltage of the control circuitan input signal from an input switch to the control circuit is useddirectly to control the application of power from the battery to theload. It follows that, although the control circuit will be disabled andwill not exhibit all its design functions when the battery voltage istoo low, the control circuit is nonetheless capable of allowing themaximum extraction of energy from the battery, when required. This canbe done without voltage sensitive parts (eg. oscillator and control ordecision making logic) of the circuit being operational.

The switching circuit may include a comparison unit for comparing thevoltage of the power source, when the power source is connected to theload, to at least one reference voltage thereby to provide an indicationof the status of the power source.

The voltage which is compared to the reference voltage is thus theclosed circuit voltage of the power source.

The switching circuit may include at least one light emitting devicewhich is energised to provide a visual indication of the said powersource status.

The switching circuit may include a memory unit for storing a measure ofthe voltage of the power source, particularly the closed circuitvoltage. This facility means that an indication of the status of thepower source is available even though the power switch is open.

The switching circuit may include a measuring unit for measuring theopen circuit voltage of the power source, ie. when the power switch isopen, and the comparison unit may be adapted for comparing the opencircuit voltage measurement to the closed circuit voltage of the powersource, and for providing a signal if the open circuit voltage dropsbelow the previously measured closed circuit voltage.

The said reference voltage may be stored in the memory unit. Preferablytwo or more reference voltages are stored in the memory unit.

The control circuit, in response to the closed circuit voltage of thepower source, may control the power switch in order to vary the dutycycle of the current which is passed to the load to achieve asubstantially constant power supply to the load.

Preferably in the case in which light emitting devices are used toprovide a visual indication of the said power source status at least oneof the said light emitting devices is used to act as a find-in-the-darkindicator by causing the said light emitting device to flash at a verylow duty cycle, at the same time continuously showing the batterystatus.

After the circuit has been powered up or down the memory unit may bereset with a fresh measurement of the prevailing open circuit voltage orof the closed circuit voltage of the power source. The control circuitmay be adapted to close the power switch automatically and for a shortperiod of time for obtaining a closed circuit voltage measurement of thepower source. This can be done for example once every 24 hours.

According to a variation of the invention the power switch is latched onor off, according to requirement, when the power source voltage is toolow to cause normal operation of the control circuit ie. when thecontrol circuit enters a non-functioning or reset state which isdependent on the battery voltage.

According to a different aspect of the invention if the switchingcircuit includes a first signal switch for on/off selection and a secondsignal switch which selects a plurality of functions then the secondsignal switch may be enabled to provide an “off” command to the controlcircuit, and hence of the load which is connected to the controlcircuit, if the second signal switch is activated a predetermined period(eg. 2 seconds) after the last operation of the second signal switch orfirst signal switch.

According to a further aspect of the invention the control circuit mayinclude a voltage regulation capability which provides a regulatedvoltage through the power switch to the load.

The switching circuit may include a timer which controls the saidpredetermined period and wherein the timer is reset each time the signalswitch is operated.

The control circuit may include an input pin or contact to which aninput signal is applied from the signal switch. This may take place inany appropriate way eg. by activating the signal switch which isconnected to the input pin or contact. Alternatively the control circuitmay detect activity by a user in any other appropriate way eg. bymonitoring activities at or to all input terminals to the controlcircuit and, each time an activity is detected, resetting the counter ortimer. Thus the delayed switch-off function will only occur if noactivity is detected for the full duration of a predetermined period(prior to the switch-off function).

The switching circuit may include a first signal switch which isconnected to an edge triggered input of the control circuit and a secondsignal switch which is connected to a state triggered input of thecontrol circuit and wherein a signal input by the first signal switchcauses the power switch to change from a state previously determined bya signal input from the second signal switch, and a signal input by thesecond signal switch does not cause the power switch to change from astate previously determined by a signal input from the first signalswitch.

In this specification the phrases “control circuit” and “microchip” areused interchangeably.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described by way of examples with reference tothe accompanying drawings in which:

FIG. 1 is a block diagram illustrating the use of a microchip forcontrolling an electrical load in accordance with the principles of theinvention,

FIG. 2 illustrates typical components of the microchip,

FIG. 3 is a simplified flow chart of a sequence of operations which canarise during the operation of the microchip,

FIG. 4 is a flow chart, similar to FIG. 3, of a slightly differentsequence of operations,

FIG. 5 is a flow chart illustrating operation of the microchip under adifferent set of conditions,

FIG. 6 illustrates a variation of the invention,

FIG. 7 is a block diagram representation of a circuit according to theinvention for controlling the supply of power from a battery powersource to a load which, in this case, is a lamp,

FIG. 8 is a more detailed representation of an integrated circuit(microchip) used in the circuit of FIG. 7,

FIG. 9 is a representation of a typical modulated signal which isdelivered by the circuit of FIG. 8 to the load,

FIG. 10 is a flow diagram representation of certain steps in theoperation of the circuit of FIG. 8,

FIG. 11 is a graphical representation of a duty cycle as a function ofvoltage for a 3.6V bulb,

FIG. 12 shows voltage error and duty cycle curves respectively as afunction of power source voltage,

FIG. 13 illustrates a variation of the invention, and

FIG. 14 shows a way of packaging the circuit of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 of the accompanying drawings schematically illustrates aswitching circuit 10 which includes a microchip or control circuit 12, abattery 14, a load 16, and a signal switch 18.

The arrangement shown in FIG. 1 is similar to what has been described inthe specification of international application No. PCT/ZA99/00107 and,for a detailed description of the operation of the arrangement,reference is made to the specification of the international application.

The switch 18 functions as an interface between an operator or anyappropriate actuating mechanism, and the control circuit 12, and henceis referred to as a man-machine-interface (MMI) This term is howeveradopted merely for the sake of convenience for, as noted, the switchcould be operated by human intervention or by any mechanism eg. aclosing door or other device which acts on the switch. The controlcircuit 12, in response to signals input from the MMI, causes power tobe applied from the battery 14 to the load 16. The load may varyaccording to application and for example may be a light bulb, a heateror the like.

FIG. 2 illustrates internal components of the control circuit 12.Voltage from the battery 14 is applied to terminals 22 and the switch 18controls the application of power to a control circuit or unit 30 insidethe circuit 12. The circuit 12 additionally includes at least a timer32, a power or load switch 34 and, optionally, a voltage regulator 36.

In general terms the control unit 30, in response to a signal from theswitch 18, causes operation of the power switch 34 and thereby connectsthe battery 14 to the load 16, or disconnects the battery from the load.The power switch 34 may or may not be part of a combined integratedcircuit with the signal switch.

The switch 18 may vary in its construction and for example, may be apush button switch ie. of momentary actuation, or a slide switch ie. onor off (in two stable states). If the control circuit 12 is fabricatedseparately from the switch then, in advance, the nature of the switchwith which the control circuit 12 is to be used may not be known. FIGS.3 and 4 illustrate different flow charts which arise through differentphases of operation of the microchip of the invention with differentrespective switch types.

Assume that the control switch 18 is a push button, ie. a push-to-makeand release-to-break, switch. Referring to FIG. 3, from an off state 40,if the switch is operated to make contact for less than a predeterminedtime interval T, say less than 0.5 seconds (step 42), then the switch isregarded as a push-to-make switch and the power is latched on (step 44).To switch the power off (step 46) contact must again be made or,alternatively, switching off can be initiated by a function of themicrochip eg. auto shut-off. These aspects are indicated by means of ablock 48 labelled “external”.

FIG. 4 illustrates a sequence of operations for a slide switch, ie. forpush-to-make and push-to-break operation. In this case, from an offstate 40, if contact is made for more than a period T of, say, 0.5seconds, (block 42A), the switch is regarded as a slide switch and poweris kept on (44) until turned off (46) by means of an external mechanism48, eg. auto shutoff or operation of the switch.

The aforementioned type of operation, with both switch types, is highlydesirable under certain conditions for example in the case of aflashlight which is hand held by a law enforcement officer. Typicallytwo distinct functions are required. The first is to have the lightswitch on while scanning an area. In this case currently availableproducts provide a push-to-make type of button and the light is on whilethe button is pushed and goes off immediately upon release. Clearly onehand is occupied full-time to have the light on.

Secondly, to switch the light on for an extended period of time, atwisting operation is required. The front or back end of the flashlightis, in some cases, turned to have a slide-switch action. This normallyrequires the use of two hands which may not be convenient.

The present example of the invention provides an elegant solution tothis type of situation. A momentary press on the switch causes theflashlight to be turned on for an extended period. Thus single handoperation can be resorted to. The light shines until the button is againpushed or auto shut-off occurs. This can be very convenient for anofficer holding a gun in one hand and wanting to switch his flashlighton for an extended period of time.

Permanent-on selection can easily be accommodated. In this case anoverride of the auto-shut off procedure is needed. This can be providedby stipulating that a on-off-on action within a short period, say withintwo seconds, is interpreted by the control unit 30 as a permanent-onmode selection. In other words three presses on a push button or threeslides of a slide switch will be interpreted as being permanently on.The sequences are indicated in FIGS. 3 and 4 respectively by the steps42, 50 and 52 and 42A, 50A and 52A respectively, resulting in each casein a permanently on state 54.

The mode selected is indicated for example by means of a visual displaydevice such as a light emitting diode 56. The LED may also be used as afind-in-the-dark facility. For example a single flash every two secondson this indicator can indicate an on state with auto shut down inoperation. Two flashes in quick succession with a delay of approximatelytwo seconds before the next two flashes may indicate a permanent onmode. More flashes in quick succession with a delay before the nextflashes may indicate other selected modes.

An important aspect of the invention is operation in a low voltageregion. A flashlight or toy (ie. the load 16) operating from the battery14, will encounter a situation in which the battery is depleted. Oftenthe battery runs down gradually with its voltage level slowly dropping.Typically the voltage may be within the operational specification of themicrochip or control circuit 12 and then it will slowly deplete anddrift out of specification.

It is desirable, under certain conditions, to give the user as much useas possible from a set of batteries and also to try and match theperformance of a mechanical switch which is not normally voltagedependent or responsive.

The microchip 12 is, in accordance with an aspect of the invention,supplied with a reset circuit 60 (see FIG. 2) which functions in amanner which is known in the art. Below a certain voltage threshold thereset circuit 60 keeps the microchip 12 in a reset or non-functioningstate. Above the threshold voltage the reset circuit 60 allows thecontrol circuit 12 to operate in accordance with its designrequirements.

In the present case a reset signal, produced by the circuit 60, is usedto connect the signal from the switch 18, ie. from the MMI, directly tothe power switch 34 when the circuit is in a reset state. In such astate the power switch conducts current when the MMI switch is closed.

The effect of this is that the auto shut off and other functions aredisabled when the voltage of the battery 14 is below designspecification. If a push-to-make switch is used to select the on statewith a single short push, the power is turned off when the reset circuitturns on. In a flashlight example however the user is able to shine thelight, although it is very dim, by keeping the push button depressed.

In the case of a slide switch the auto shut off and other functions areagain disabled at the time the reset circuit turns on but, with theflashlight in an on state, the flashlight stays on as the battery 12gets fully depleted. The flashlight will switch off once the slideswitch is moved to a non-contact position. The find-in-the-dark andsimilar functions are disabled since the relevant portion of themicrochip is not operational when the reset circuit 60 is turned on.

In other embodiments the power switch 34 may be latched on/off when areset state is entered. Latching on may be important in cases whencontinuous operation to the lowest possible voltage is required eg. witha flashlight. An off selection may be important in a case where avoltage which is too low may damage the product eg. with an electricmotor.

FIG. 5 illustrates, in flow chart form, the aforementioned sequence ofoperations. The voltage 70 of the battery 14 is monitored (step 72). Ifthe voltage is within specification then the control circuit 12functions normally (step 74). If the voltage drops below specificationthen the reset circuit 60 detects this (step 76) and the power switch 34is then made directly responsive to the MMI switch 18 (step 78).Depending on requirement and in particular on the nature of the load 16the power switch 34 can be latched on or off (step 80)

Assume that the control circuit 12 is responsive to the switch 18 and toa second switch 18A shown in dotted lines in FIG. 1. In other words theMMI has two signal switches. The first switch may for example may beused for on/off selection while the second switch may be used to selectthe mode of operation and, when pressed, may step sequentially throughvarious modes such as a first level of dimming, a second level ofdimming, fast flash, slow flash, etc, in a cyclical manner.

A user may get confused with the two switches. Assume for example that adimming mode is selected and operated for a period of time. If the userwants to switch the flashlight off but inadvertently presses the modeswitch 18A instead of the first switch 18 the flashlight again startsflashing. This may be bothersome and confusing for some users. Underthese conditions the control unit 30 may detect operation of the modeselection switch 18A, when operated after a predetermined period of,say, 10 seconds, and change its function so that such operation resultsin an off function ie. the load 16 is disconnected from the battery 14.

In another embodiment of the invention the control circuit 12 includes avoltage regulator 36 This is used to provide a regulated voltage,derived from the battery 14, to the load 16. This may be highlydesirable under certain conditions for it can be used to prevent anovershoot of the voltage, applied to the load, when the switch 34 isturned on and can enable much longer operation at an optimal voltage.

Assume for example that a flashlight operating from a 6V battery packhas an output from the current switch regulated to 4.5V. As the batterygets depleted from its initial 6V the light will have optimal andconstant performance until the battery voltage drops to 4.5V. When thebattery voltage drops below this level the current supply to the loadwill gradually diminish and will continue to diminish until the batteryis totally exhausted. This feature helps in the design and choice ofbulb for use in the flashlight and also helps to improve the life spanof the bulb.

In a further embodiment the switch can provide for flashlight operationat a regulated voltage, for example a four cell flashlight (6V) may beregulated to work at 4V with a suitable bulb for the 4V output. If theuser wants to have brighter light for a short period, the unregulatedvoltage can be applied to the bulb or, if the voltage has dropped, astep-up can be performed to yield a very bright light.

The invention is frequently described herein with reference to theoperation of a light bulb. It is to be understood that this is only byway of example and that the invention can be used with any other loadeg. a heater, motor or any other electrical device.

FIG. 6 of the accompanying drawings illustrates a microchip 110according to another form of the invention which is supplied withelectrical power from a source 112. The microchip includes a powercontrol switch 114, a timer 116 and a control circuit 118. A load 120 isconnected to the microchip. The switch 114, under the control of thecontrol unit 118, controls the connection of the power source 112 to theload 120 in accordance with various criteria.

A switch 122 is used to turn the microchip on or off. This switch givesto the user overall control of the microchip so that the microchip canbe enabled or disabled.

An optional second switch 124 is used as a mode selector switch. Thisenables a user to select different modes or functions of the microchipso that the load 120 is controlled, or caused to operate, in a differentway.

The drawing illustrates a third switch 126 which is referred to as anactivity detector switch. Although shown as a switch this could, in somesituations, be a symbolic notation only for activity of the user couldbe detected in any appropriate way by making use of any suitablemonitoring system.

The switch 126, which is used to detect activity, can be one of a numberof switches which are used to control operation of the control unit 118.Alternatively the switch 126 can be operated in a particular way orsequence to provide different command signals to the control unit 118 sothat the control unit can function according to requirement.

Assume for example that the load 120 is an animated toy or similardevice which is powered by the power source 112 in a manner which isdetermined by the operation of the switch 126 and, where appropriate, bythe switch 124. Each time the switch 126 is activated the timer 116 isreset. Thus the switch 114 will remain closed and the load 120 will beenergised while there is activity at the switch 126. Once the activityceases the timer 116 starts its timing period. If, during this period,there is no activity at the switch 126 then the timer 116 will completeits timing period and, at the end thereof, will cause the switch 114 toopen circuit. In other words the delayed switch-off function will beimplemented a predetermined period, which is determined by the timerinterval, after the last activity is detected by the control unit 118.

Any appropriate means may be used for detecting activity relating to themicrochip. As stated, the switch 126 may be operated in a variety ofdifferent ways in order to exert different control functions or,alternatively, the switch 126 may be one of a plurality of similarswitches each of which exerts a respective control function. Dependingon the protocol a switch which is opened or closed, can be detected asactivity. Thus the timer could be reset by a high input signal, a lowinput signal or a transition signal in a chosen direction eg.low-to-high, or high-to-low, or any combination thereof.

FIG. 7 of the accompanying drawings illustrates a circuit 210 forcontrolling the supply of power from a battery power source 212 to aload 214. The nature of the load may vary from case to case. In thefollowing description the load 214 is described as being an electriclight bulb but this is given merely by way of a non-limiting example.

A number of signal switches 216 are connected to the circuit and areused for controlling its operation. A number of light emitting diodes218 are connected to the circuit 210 and are used for indicating certainaspects of the operation of the circuit and for providing an indicationof the status of the battery 212, in the manner which is describedhereinafter.

FIG. 8 is a more detailed block representation of the circuit 210. Thecircuit 210 is an integrated circuit and includes a control unit 220, ananalogue to digital converter 222, an input and output unit 224 whichfunctions as an interface, timing, reset and oscillator modules 226, 228and 230 respectively, a current switch 232 and a memory module 234.

The switches 216 are used for actuating the control unit 220 through themedium of the input and output unit 224 Selected information generatedby the control unit 220 is transferred to the light emitting diodes 218through the medium of the input and output unit 224.

The voltage from the power source or battery 212 is measured andconverted to a digital format by the analogue to digital converter 222.The converter also detects whether the current switch 232 is opened orclosed when the battery voltage is measured and, in this way, is capableof providing a digital measurement of the open circuit voltage or theclosed circuit voltage of the battery 212.

Once the input voltage has been digitally measured its value, referredto as V_(D) herein, is transferred to the control unit 220.

Pre-programmed reference voltages are stored in the memory module 234.The number of reference voltages which are stored is determined by thenumber of categories in which the battery voltage can be classified. Forexample a three category indication of good, medium and bad requires atleast two reference values V_(G) and V_(M). The battery indications areas follows depending on the relative voltage levels:

-   V_(D)≧V_(G)—“good” indication;-   V_(G)>V_(D)≧V_(M)—“medium” indication; and-   V_(D)<V_(M)—“bad” indication.

By actuating a selected switch 216 the control unit 220 is caused tooperate, substantially in the manner which is described in thespecification of international application No. PCT/ZA99/00107, and theswitch 232 is closed thereby to connect the battery 212 to the load 214.As indicated the battery voltage is measured and compared to thereference voltages stored in the memory module 234. An indication isthen substantially immediately given of the status of the battery viathe appropriate light emitting diode or diodes 218 which are activatedin a predetermined manner.

The measurement of the closed circuit voltage of the battery can bestored in the memory module 234. When the battery is disconnected fromthe load the open circuit battery voltage can be measured automaticallyby the action of the control unit, at regular intervals, and compared tothe closed circuit voltage stored in the memory module 234. If the opencircuit voltage drops below the stored value of the closed circuitvoltage the category (good, medium, bad) in which the battery isclassified can be altered to reflect the change in the batterycondition.

The timing unit 226 is used to control the intervals at which thebattery voltage is measured and for controlling the frequency at whichthe light emitting diodes 218 are pulsed. It is to be noted that whenthe switch 232 is open at least one of the diodes 218 may be pulsed at avery low duty cycle, which is controlled by the timing unit 226, toprovide a find-in-the-dark facility. The low duty cycle ensures that thepower consumption of the flashing LED is kept to a low value.

The control unit is capable of modulating the power supply, underappropriate conditions, to ensure that the power which is supplied tothe load 214 is kept substantially constant as the battery voltage dropsdue to power consumption. Assume for example, as is shown in FIG. 11,that the voltage of the battery 212 is initially at 8V and that the load214 is a 3.6V bulb. As the battery voltage drops due to the charge inthe battery being diminished through usage the duty cycle of the powersupplied to the load increases. In the range of from 8V down to 3.6V,the duty cycle is non-linear with respect to voltage and, once thevoltage crops to 3.6V, the duty cycle is 100%.

In the case of a light bulb it is important to note that the light bulbwill give substantially constant illumination if its average powerconsumption is constant. FIG. 9 illustrates a way in which the batteryvoltage can be modulated or switched by the oscillator 230 actingdirectly or indirectly on the current switch 232. In this instance, withthe battery voltage at 6V, pulses with a duration of approximately 0.36ms are generated at a frequency of approximately 1 kHz which iscontrolled by the oscillator unit 230.

Apart from errors based on resolution it is only after the batteryvoltage drops below 3.6V that the light bulb illumination starts todegrade. The acceptable margin for error determines the accuracyrequired for measuring the voltage across the battery as well as theresolution with which the duty cycle can be constructed.

FIG. 12 shows a graph of the error levels caused by digital rounding ifthe battery voltage (V_(D)) is measured to an accuracy of 0.4V and a 5%resolution is used for the duty cycle construction. Clearly thisaccuracy is a function of complexity and cost, and can be increasedwithin practical limits.

The functions of battery status and constant illumination require avoltage measurement of the closed circuit voltage of the battery ie.when the load is connected to the battery. From a cost andimplementation perspective these functions are logically provided by theintegrated circuit 210. On the other hand if the product with which thecircuit is used, eg. a flashlight, has a find-in-the-dark feature, eg.an LED which flashes at a low duty cycle, then it stands to reason thatthe battery life indication can be provided by the find-in-the-dark(indicator to reduce the component count.

The effect of constant illumination can also be achieved by stepping upthe input voltage to a higher voltage level. This can be implemented byusing various DC-to-DC step-up converter techniques which are known inthe art. The advantage of a step-up approach means that higherillumination levels are achievable than what would otherwise be the casewhen the battery voltage drops. This however would be at the cost ofadditional components. One can also make use of a step down converter toachieve substantially constant illumination.

The switching rate of the switch, ie. the modulation rate of the powersupplied to the load, must be such as to avoid overheating during the onpart of the cycle and to prevent flickering occurring which is visibleto the human eye. On the other hand with every switching action somepower losses occur. Thus lower modulation frequencies are more efficientand components are more feasible. For Xenon bulbs a 1 kHz switching rateappears to be a good compromise.

FIG. 10 is a simple flow diagram representation for the on/off functionof a flashlight with a continuous indication of battery status.

When the batteries are inserted into the flashlight (300) flags in thecontrol unit are set (310). At this stage the circuit 210 does not haveinformation about the battery status and the open circuit voltage of thebattery must be measured. Optionally the flashlight is momentarilyturned on to measure the closed circuit voltage. This enables thebattery life indicator flag (BLI flag) to be set in accordance with themeasured voltage.

A block 315 indicates that the load is switched off at this time.

If a find-in-the-dark (FID) function is implemented the integratedcircuit 210 causes the corresponding LED (good, medium or bad) to flasheven though the bulb 214 is not illuminated (325).

Upon user activation (320) the inputs 216 to the circuit 210 becomeactive. Within normal state of the art practices like debouncing etc.the control unit 220 turns the current switch 232 on. In order toprotect against overheating the calculation of the correct duty cyclefor the applicable input battery voltage must be made faster than thelowest duty cycle, or the duty cycle must be set to its lowest (safest)level (330), so that the duty cycle calculations can be done. Thereafterthe correct duty cycle, which determines the illumination of the bulb214, can be selected (335).

Once the closed circuit voltage has been measured and converted theresulting digital value V_(D) can be compared against reference valuesin order to categorize and indicate the status of the battery (340)(BLI=battery life indicator).

The control unit 220 performs the functions up to step 345 in a shorttime. A check is constantly required (step 345) to see if the user hasreleased the appropriate input switch 216. If not the steps 335, 340 and345 are repeated. The input switch 216 is monitored more actively thanthe voltage levels in order to conserve power. Once the release of theinput switches has been detected (345) the control unit 220 proceeds tocheck the voltage levels to adjust for a possible drop in supply voltage(350) and to check the input switches 216 (355) for a terminate command.If a terminate command is received the control unit 220 turns thecurrent switch 232 off (block 360).

Upon the subsequent release of the input switch 216 by the user thecontrol unit 220 again checks the input switches (365) for an activationcommand and performs the find-in-the-dark (FID) function using thebattery life indicator (BLI) category determined during the previous onstate. The control unit 220 also monitors the battery voltage level todetermine if the open circuit voltage has dropped lower than thepreviously measured closed circuit voltage. As has been stated this mayrequire a change in the indicated category of the battery status.

FIG. 13 illustrates a further aspect of the invention. In this case useis made of a control circuit or microchip 400 substantially of the kinddescribed hereinbefore which is supplied with power from a battery 402.The control circuit has three signal switches 404, 406 and 408respectively connected to respective inputs 404A, 406A and 408A. Anoutput from the control circuit controls a current switch 410 which isused to connect power from the battery 402 to a load 412.

In this example of the invention the inputs 404A and 406A are referredto as main and mode inputs respectively and are edge triggered inputs.In other words these inputs are designed to work with non-latching inputdevices 404 and 406. As such, a first momentary activation of therespective signal switch 404 or 406, as the case may be, causes thecontrol circuit 400 to interpret the input as an “on” command and asecond activation will be interpreted as an “off” command.

By way of contrast the input 408 is a state triggered input. Thus a highinput level is interpreted as an “on” command while a low input level isinterpreted as an “off” command.

It is to be borne in mind that the levels referred to are merely by wayof example and that the “on” and “off” commands can result from inputsignals at levels which are opposite to or different from what has beendescribed.

Assume for example that the signal switch 408 is connected via a signalwire to the input 408A. The signal switch 408 may for example be acontroller, such as a door-operated switch, in a vehicle which controlsillumination inside the vehicle. For example when a door of the vehicleis opened the load 412, which is a lamp in the vehicle, is energised bythe battery 402. In this configuration the switch 404 is installed, forexample, at the point of illumination ie. near to the load 412.

The aforementioned configuration eliminates the requirement for acurrent carrying lead or wire from the signal switch 408 to the load 412ie. a wire which can carry the full load current drawn by the lamp 412.This reduces the weight and cost of the lighting installation.

It is important that a signal from the switch 404 to the input 404A canoverride an input from the signal switch 408 to the input 408A, but notvice versa. Thus with the light 412 on due to a high input provided bythe signal switch 408 an activation of the switch 404 will turn thelight off. However if the light is on due to a command which haspreviously been input from the switch 404 to the input 404A a high levelresulting at the input 408A by activation of the signal switch 408 willnot affect the light condition and neither will a subsequent low inputat the input terminal 408A.

The control circuit 400 will turn the light 412 off after an on periodof a defined duration, for example 30 minutes or an hour, depending onrequirement, irrespective of which input caused the on condition.

The mode input 406A exhibits the same protocol or hierarchy as does themain input 404A with regards to the input 408A. Thus a signal input viathe switch 406 will override a state previously determined by an inputfrom the switch 408, but not vice versa. A signal applied to the modeinput can be used to reduce the power to the light by causing the dutycycle to be reduced and this will result in the light dimming.

A further aspect of this invention is shown in FIG. 14 which depicts,somewhat schematically, a housing 500 of a conventionalelectromechanical switch 502 operated via a button 504 The microchip orcontrol circuit 12, eg. of the type shown in FIG. 2, is packaged insidethe housing 500. The switch 502 acts as a signal switch 18 and controlsthe connection of a power supply 506 to a load 508. In other respectsthe circuit acts in the manner described hereinbefore. The switch 502may vary according to requirement and for example may be a pushbuttonswitch (push-to-make release-to-break; eg. Carling switch P27L13L), atoggle switch (single pole double throw; eg. Arcolectric V1722RO) or arocker type switch (eg. Arcolectric PLC1522AA) Using the two wire modethe switch can be pin compatible albeit polarity sensitive, but canprovide intelligent decision making actions.

1. A switching circuit for controlling the supply of power from anexhaustible power source to a load which includes a control circuit, apower switch which is controlled by the control circuit in response tosignals from a man-machine-interface (MMI) and said power switch, whenclosed, connects the power source to the load, the MMI comprising atleast a switch connected to at least one signal switch input of thecontrol circuit, the switch does not form a serial element in a circuitthat transfers power from the power supply to the load and wherein thecontrol circuit further provides at least one function selected from thefollowing functions: (a) a power source level indication which isdetermined by comparing a fixed reference voltage to a closed circuitsupply voltage and which is displayed to a user, even at times when theload has not been activated through the MMI; and (b) a function whereinthe control circuit compares the voltage of the power source to a fixedreference voltage and, in response to the comparison, controls a dutycycle of the power switch to provide a substantially constant supply ofpower from the power source to the load over a voltage range of thepower source.
 2. The switching circuit according to claim 1 wherein saidfunction wherein the control circuit compares the voltage of the powersource to a fixed reference voltage is selected and the level of saidsubstantially constant supply of power is determined in accordance witha level selected through the MMI.
 3. The switching circuit according toclaim 1 wherein the control circuit monitors the voltage of the powersource and when the power source voltage drops below an operatingvoltage of at least one element of the control circuit, the power switchis controlled directly by a signal at the signal switch input to connectthe power source to load.
 4. The switching circuit according to claim 1wherein the power source level indicator also functions as afind-in-the-dark indicator.
 5. The switching circuit according to claim1 wherein the power source level indication is additionally based on ameasurement of the level of the supply voltage when the power switch isnot conducting.
 6. The switching circuit according to claim 1 whereinthe load is an electric motor and the duty cycle of the power switch iscontrolled to achieve a substantially constant operation of the motorover a range of power source levels.
 7. The switching circuit accordingto claim 1 wherein the load is a lighting element and the duty cycle ofthe power switch is controlled to achieve a substantially constantillumination level of the lighting element over a range of power sourcelevels.
 8. The switching circuit according to claim 1 wherein the powersource level indication additionally gives an indication of a selectedfunction or mode of the control circuit.
 9. The switching circuitaccording to claim 1 wherein said circuit is packaged within a housingof a conventional electromechanical switch.
 10. The switching circuitaccording to claim 1 wherein the substantially constant power suppliedto the load is achieved by stepping up the input voltage to a highervoltage for the load.
 11. The switching circuit according to claim 1wherein the control circuit provides at least one further function inresponse to a sequence of activation signals at the signal switch input.12. The switching circuit according to claim 11 wherein the sequence ofactivation signals is changed dynamically to select “off” on the nextactivation or deactivation signal once any specific selection from thefunctions has been active for a predetermined period of time.
 13. Theswitching circuit according to claim 1 wherein the control circuitmonitors the voltage of the power source at predetermined intervals inthe “on” and in the “off” state of the power switch.
 14. The switchingcircuit according to claim 13 wherein a load is automatically connectedto the power source before the voltage of the power source is measured.